Reduced mesophyll conductance under chronic O 3 exposure in poplar reflects thicker cell walls and increased subcellular diffusion pathway lengths according to the anatomical model.

Ozone (O ₃ ) is one of the most harmful and widespread air pollutants, affecting crop yield and plant health worldwide. There is evidence that O ₃ reduces the major limiting factor of photosynthesis, namely CO ₂ mesophyll conductance (g _m ), but there is little quantitative information of O ₃ -caused changes in key leaf anatomical traits and their impact on g _m . We exposed two O ₃ -responsive clones of the economically important tree species Populus × canadensis Moench to 120 ppb O ₃ for 21 days. An anatomical diffusion model within the leaf was used to analyse the entire CO ₂ diffusion pathway from substomatal cavities to carboxylation sites and determine the importance of each structural and subcellular component as a limiting factor. g _m decreased substantially under O ₃ and was found to be the most important limitation of photosynthesis. This decrease was mostly driven by an increased cell wall thickness and length of subcellular diffusion pathway caused by altered interchloroplast spacing and chloroplast positioning. By contrast, the prominent leaf integrative trait leaf dry mass per area was neither affected nor related to g _m under O ₃ . The observed relationship between g _m and anatomy, however, was clone-dependent, suggesting that mechanisms regulating g _m may differ considerably between closely related plant lines. Our results confirm the need for further studies on factors constraining g _m under stress conditions.
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